Device for the hold-down and controlled release of space satellites installed on launchers and loads installed on space satellites

ABSTRACT

A device for the hold-down and controlled release of space satellites installed on launchers and loads installed on space satellites, includes a blocking member made up of segments radially displaceable from a mutually approached retaining position to a mutually spaced-apart release position of a connecting screw. In order to control the radial displacement of the segments between the retaining position and the release position there are provided a rotatable element coaxial with the blocking member, an articulated mechanism which operatively connects the rotatable element with the segments of the blocking member, and linear motors for driving the controlled rotation of the rotatable element and, through the articulated mechanism, the simultaneous displacement of the segments of the blocking member.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/380,428 filed Dec. 15, 2016, which claims priority to Italian PatentApplication No. 102015000084151 filed on Dec. 16, 2015, the entiredisclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention regards devices for the hold-down and controlledrelease of space satellites installed on launchers and loads installedon space satellites. Devices thus made have the function of retainingthe load during the launch and obtaining lift-off into space.

STATE OF THE PRIOR ART

Various devices, referred to as HDRM (“Hold-Down Release Mechanism”),that utilise pyrotechnic charges, devices for cutting wires, shapememory alloy wires, electromagnetic wires, thermal fuse thrusters,thermal knives, magnetic retentions and piezoelectric actuators, havebeen proposed.

These devices share one or more of the drawbacks summarised below:

-   -   actuation characterised by high mechanical shock, which may        potentially affect the functionality of the instruments on board        the satellite,    -   release of material (solid, liquid or gaseous) that is pollutant        and potentially harmful for the on-board optical apparatus,    -   storage and use challenges,    -   need to reset after every actuation and this having to be done        by the manufacturer, thus affecting costs, times and        reliability,    -   need to replace parts after every actuation, equally affecting        costs, times and reliability,    -   limited number of actuations and ensuing limitation of        pre-flight tests,    -   expensive verification of the hold-down pre-load level and the        ensuing occurrence of uncertainty elements.

There have also been proposed HDRM devices in which the engagement of ahold-down screw requires a nut made up of segments radially displaceablefrom a mutually approached retaining position to a mutually spaced-apartrelease position of the screw, and in which there are provided actuatormeans for controlling the radial displacement of the nut segments fromthe retaining position to the release position.

Systems of this type are described and illustrated, for example, indocuments EP-1680607, in which the nut segments are retained by a camring whose rotation, driven by linear or rotary electric motors or bypre-loaded spring plungers, enables opening the segments; EP-2279120, inwhich the nut segments can be elastically diverged and they are radiallyfastened by means of a hollow tensioner whose release is controlled byan actuator; EP-2605968, in which the nut is replaced by a sleeve formedby at least two parts that delimit a seat in which there is engaged thespherical head of a connection bar, and the two parts are fastened toeach other through a spiral plate connected to a release device;GB-1147227, in which there is used an annular retaining piston whichradially holds the nut segments and whose axial displacement, carriedout through pressurised gas generated by explosive charges or evenhydraulically, enables diverging the segments; GB-2373554, whichprovides for obtaining the diverging of the nut segments through theaxial displacement of a containment sleeve formed with inner recesseswhich enable the radial opening movement of the segments, and thedisplacement of the containment spring is obtained hydraulically; U.S.Pat. No. 3,813,984, in which the diverging of the nut segments isobtained through the axial displacement of two bushings provided withinternal recesses and driven hydraulically in opposite directions; U.S.Pat. No. 3,926,090, in which the diverging of the nut segments is alsoobtained through a plunger whose movement activates a kinetic energydissipator to reduce the actuation mechanical shock; U.S. Pat. No.5,245,738, in which the nut with segments is contained in a bushing alsoof the type provided with segments in turn enclosed in a spiralretaining wire whose loosening and whose ensuing expansion are driven bya flexible hollow transmission; U.S. Pat. No. 6,352,397, which providesfor a rotor normally blocked in rotation and cooperating with a statorhaving an axial support inclined surface of an end of the nut segments,and an axial thruster which acts on the other ends of the segmentscausing the diverging thereof so as to free the rotor in rotation tofree the screw; U.S. Pat. No. 6,525,920, in which the head of theconnection screw is held by a body (“split spool”) made up of twosymmetric elements pressed against each other by a torsional spring,which is held by a metal wire which is broken due to the Joule effect toenable the opening of the spring and the separation of the two elementsto free the screw.

These solutions are also generally affected by one or more of thedrawbacks listed above, thus the use thereof is considerably limited.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome the aforementioneddrawbacks and such object is attained by a device as defined in thepre-characterising part of claim 1, whose peculiar feature lies in thefact that the actuator means comprise an element rotatably coaxial withrespect to the blocking member, an articulated mechanism whichoperatively connects said rotatable element with said segments of theblocking member, and drive means for driving the controlled rotation ofsaid rotatable element and, through said articulated mechanism, thesimultaneous displacement of the segments of the blocking member betweensaid retaining and release positions. The articulated mechanismcomprises a plurality of connecting rod and crank units, each of whichconveniently defines a knee connected to the rotatable element.

According to a first embodiment of the invention, the rotatable elementis an elastic ring forcedly positioned between stationary retainingmeans and the drive means are linear motors, for example of thepiezoelectric type, which act in radial direction on the elastic ringbetween the stationary retaining means to perform the sequentiallocalised micro-deformations thereon.

The coupling between the elastic ring and the connecting rod and crankmechanisms imparts the coordinated and simultaneous rotation of the pinsbetween the connecting rods and cranks around the hinges of the cranks,thus enabling the radial excursion thereof with respect to the axis ofthe device. This leads to the coordination and simultaneousness of themotion of the connecting rod and crank mechanisms, and thus of the nutsegments, between the retaining position and the complete releaseposition of the screw.

In variants of the invention the drive means consist of elements made ofshape memory material, more in particular wires eccentrically connectinga rotatable member with a stationary member, operatively connected to aheating system for example by Joule effect so as to be subjected to atransformation from the martensite phase to the austenite phase of theshape memory material. The transformation produces a shortening of thewires which induces a rotation of the rotatable member relative to thestationary member and the congruous simultaneous actuation of theconnecting rod and crank units, with the ensuing displacement of thesegments of the blocking member from the retaining position to therelease position.

The invention enables attaining considerable advantages, as listedbelow:

-   -   release without generating mechanical shocks,    -   high level of applicable pre-load,    -   use of standard screws,    -   release without generating materials,    -   unlimited number of retaining and release actuations,    -   no operation to reset or replace parts is required,    -   ease of application of means for detecting the applied pre-load,    -   exclusive use of patented actuation technology, based on use of        piezoelectric linear micro-actuators or, alternatively, elements        made of shape memory material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described in detail, purely by way ofnon-limiting example, with reference to the attached drawings, wherein:

FIG. 1 is a schematic perspective view of a device according to theinvention for the hold-down and controlled release of space satellitesinstalled on launchers and loads installed on space satellites,

FIG. 2 is a partly exploded view of FIG. 1,

FIG. 3 is a bottom and further exploded perspective view of the device.

FIG. 4 is a perspective view and in larger scale of a detail of thedevice represented in the retaining configuration,

FIG. 5 is a view analogous to FIG. 4 showing the same detail in therelease configuration,

FIGS. 6 and 7 are two views analogous to FIGS. 4 and 5, respectively,with the addition of a further component of the device,

FIGS. 8 and 9 are two perspective views that are partial, schematic andin larger scale, showing a detail of FIGS. 6 and 7 respectively in theretaining configuration and in the release configuration of the device,

FIG. 10 is atop perspective view and in larger scale of a componentillustrated in FIG. 3,

FIGS. 11, 12, 13 and 14 are diagrams—in plan view—exemplifying theoperation of the device according to the invention when passing from theretaining configuration to the release configuration,

FIGS. 15 and 16 are two cross-sectional views of a first embodiment of acomponent of the device according to the invention, respectivelyrepresented in the retaining configuration and in the releaseconfiguration of the device,

FIGS. 17 and 18 are two views analogous to FIGS. 15 and 16, showing avariant,

FIGS. 19 and 20 are two simplified perspective views showing the deviceaccording to the invention equipped with the component of FIGS. 17 and18, respectively represented in the retaining configuration and in therelease configuration of the device,

FIG. 21 shows an enlarged scale of the detail of FIG. 17,

FIGS. 22 and 23 are two dorsal perspective views of a first variant ofthe device according to the invention, respectively represented in theretaining configuration and in the release configuration of the device,

FIGS. 24 and 25 are two front views respectively of FIG. 22 and FIG. 23,

FIGS. 26 and 27 are two dorsal perspective views of a second variant ofthe device according to the invention, respectively represented in theretaining configuration and in the release configuration of the device,and

FIGS. 28 and 29 are two front views respectively of FIG. 26 and FIG. 27.

DETAILED DESCRIPTION OF THE INVENTION

Initially with reference to FIGS. 1 and 2, the device according to theinvention essentially comprises a body made up of a pair of juxtaposedplates respectively outer 1 and inner 2 between which there is arrangedan articulated mechanism indicated in its entirety with 3 and providedto control the radial displacement of radially displaceable segments 4of a blocking member constituted by a threaded nut 5 cooperating with aconnecting screw 6. Typically, the outer plate 1 is designated to befixed to a space satellite while the screw 6 will be fixed to a loadthat will be released by the satellite.

As better observable in FIGS. 4 and 5 the articulated mechanism 3comprises a triad of connecting rod 7 and crank 8 units in which eachconnecting rod 7 is articulated—on one side—to a respective segment 4 ofthe nut 5 by means of a pin 9, and—on the opposite side—to the crank 8by means of a pin 10.

Each crank 8 is oscillatingly supported between the plates 1 and 2 bymeans of a respective pin 11, and the angular excursion thereof islimited by means of a pair of abutments 12 arranged on opposite sideswith respect to the relative connecting rod 7.

The three connecting rod 7 and crank 8 units can be displaced in acoordinated and simultaneous fashion from the position represented inFIG. 4, corresponding to an approached condition of the segments 4 ofthe nut 5 in which the connecting rods 7 are substantially aligned withthe respective cranks 8, and the configuration represented in FIG. 5 inwhich the cranks 7 form a knee with the respective cranks 8 and thesegments 4 of the nut 5 are radially spaced from each other. The firstconfiguration evidently corresponds to the engagement of the screw 6 inthe nut 5, while the second corresponds to the release of the screw 6.The radial displacement of the segments 4 of the nut 5 between themutually approached position and the mutually spaced position is guidedby means of reliefs 14 projecting from the plate 1 and in whichappendages 15 of such segments 4 carrying the pins 9 are slidable.

The actuation of the articulated mechanism 3 to obtain the displacementof the segments 4 of the nut 5 between the mutually approached positionsand mutually spaced positions is obtained by means of a rotatableelement, coaxial to the nut 5, constituted by an elastic ring 16positioned outside the plate 2, on the side opposite to the plate 1.FIGS. 6 and 7 represent this ring 16 having the plate 2 fictitiouslyremoved, for the sake of illustration simplicity.

The ring 16 is formed at the surface thereof faced towards the plate 2with a triad of pairs guide reliefs 17, better observable in FIGS. 8 and9, in each of which there is freely inserted the pin 10 of a respectivecrank 8, passing through a corresponding opening of the plate 2.

The ring 16 is housed inside a casing 18 formed by an annular element 19to which there are fixed the free ends of the pins 11, also projectingthrough the plate 2, and a cover 20.

The ring 16 is forcedly inserted into the annular element 19, whoseinner surface is given by the intersection of six arcs, centred on anequal number of different positions, which determine six lobes of twodistinct forms: a thrust form and a block form, alternating with respectto each other. The lobes with block form, with respect to which the ring16 is positioned forcedly but rotatably for a limited angular width, areschematised as locking pins 22 in FIGS. 11-14, to be addressedhereinafter.

Inside the ring 16 there is arranged a body 21 formed with a radial ofseats bearing a first triad of linear motors 23, radially positionedbetween each pair of lobes or locking pins 22, and a second triad oflinear motors 24 alternated with the linear motors 23 and radiallypositioned angularly with respect to the lobes or locking pins 22.

The linear motors 23 and 24 are conveniently micro-actuators ofpiezoelectric type, and they will be hereinafter respectively referredto as thrust piezoelectric actuators 23 and block piezoelectricactuators 24.

Between the block piezoelectric actuators 24 and the ring 16 there areconveniently interposed inserts 26 whose surface is treated so as tomaximise the coefficient of friction.

The assembly constituted by the elastic ring 16, by the locking pins 22and the thrust 23 and block 24 piezoelectric actuators constitutes amotor with linear actuators substantially of the type described andillustrated in document WO-2014/013442A1 granted to PHI DRIVE S.r.l. andwhose technology is exclusively licenced to the Applicant of the presentpatent for the application subject of the present invention.

However, it should be observed that the actuation system of the segments4 of the nut 5 by means of the articulated mechanism 3 could differ fromthe one described and illustrated herein by way of non-limiting example,and for example consist in the mechanism according to documentWO-2014/013443 also granted to PHI DRIVE S.r.l., or any functionallyequivalent mechanism as long as suitable to control the motion of thesegments 4 of the nut 5 through interconnected articulations. Inaddition, the thrust piezoelectric actuator 23 and the blockpiezoelectric actuator 24 may be replaced by linear micro-actuators ofdifferent type, as long as capable of operating as describedhereinafter.

The screw 6 coaxially traverses the casing 18, the elastic ring 16, thebody 21 and the plate 2 to be screwed into the nut 5 in the radiallyapproached condition of the segments 4 thereof.

Given that the device according to the invention is normally providedfor multiple use with identical devices, a pre-load sensor 21 is alsooptionally applied outside the casing 18 with the aim of verifying thehomogeneity with which different devices are pre-loaded, so as toguarantee the simultaneous release thereof. Such pre-load device is ofthe generally known type, for example with load cells, and thus will notbe described in detail herein for the sake of conciseness.

The device according to the invention operates as follows.

The radial movement of the segments 4 of the nut 5 is operated throughconnecting rod 7 and crank 8 articulations interconnected to each otherthrough the elastic ring 16. Such ring 16 intercepts, through the guidereliefs 17, the corresponding pins 10 of the knee between eachconnecting rod 7 and the relative crank 8: this coupling imparts thecoordinated and simultaneous rotation of the pins 10 around the pins 11,without preventing the radial excursion of the connecting rods 7 withrespect to the axis of the nut 5. Such angular excursion, limited by theabutments 12, is of a few degrees and this leads to a correspondingradial translation of the appendages 15 of the segments 4 of the nut 5between the mutually approached position thereof, represented in theFIGS. 4 and 6 and corresponding to the retaining of the screw 6, and theradially spaced position represented in FIGS. 5 and 7 and correspondingto the release of the screw 6.

The rotation of the elastic ring 16 that generates the radial movementto open and close the segments 4 of the nut 5 is carried out, withreference to the diagrams of FIGS. 11 to 14, as follows.

The operating principle, described in detail in the previously mentioneddocument WO-2014/013442A1, is based on the sequential and controlledapplication of micro-deformations to the elastic ring 16, staticallyforced between the lobes of the annular element 19 schematised by thelocking pin 22. Due to such aforementioned forced positioning and thelobe-like configuration of the annular element 19, the elastic ring 16is deformed generating three lobes, as schematised in FIG. 11. At theapex of each lobe there corresponds a respective thrust piezoelectricactuator 23.

Radially pressing one of these lobes, by actuating one of the thrustpiezoelectric actuators 23 as represented in FIG. 12, such lobe isaccentuated and the corresponding locally deformed portion istransferred, reducing at least one of the other two lobes, through amicro-sliding of the ring 16 at one of the locking pins 22 adjacent tothe lobe in question, whose corresponding block piezoelectric actuator24 remains inactive. Simultaneously, block piezoelectric actuators 24juxtaposed with respect to the other two locking pins 22 locally blockthe elastic ring 16, preventing the micro-sliding thereof. Selectivelysynchronously providing the thrust piezoelectric actuators 23 and theblock piezoelectric actuators 24 with a suitable sequence and frequency,through a suitable electronic control system, enables obtaining therotation of the ring for example as represented in FIGS. 13 and 14 inwhich, like in FIG. 12, the arrows indicate the operating state of thepiezoelectric actuators 23 and 24.

To each rotation (in one direction or in the opposite direction) of theelastic ring 16 there corresponds an angular displacement of the pins 10of the connecting rod 7 and crank 8 units, and thus a correspondinglinear displacement of the segments 4 of the nut 5.

The actuation of the segments 4 by means of the articulationssynchronously driven by a rotary actuator constitutes the uniqueness ofthe device according to the invention, due to which it is possible toopen the nut 5 to release the pin 6 through a perfectly synchronousmovement of the segments 4 thereof. The device is perfectly reversible,in that the nut 5 can be opened and closed by inverting the angularmotion of the elastic ring 16, and it can thus be used for an unlimitednumber of actuations without requiring particular reset operations.

A further advantage of device according to the invention lies in thefact that it operates in open loop without requiring encoders or otherpositional control devices.

The motion of the segments 4 of the nut 5 occurs at a low speed, in acoordinated and repeatable fashion. Repeatability guarantees opensimultaneousness, while the slowness of the motion enables nullifyingthe axial pre-load of the screw 6 before the separation thereof from thenut 5, thus preventing the generation of mechanical shocks due to aninstantaneous freeing of the pre-load energy.

By way of example, it should be observed that an M8 screw with a 50 mmnon-threaded section extends by about 0.15 mm when stressed with a 30 kNpre-load. This extension is completely recovered by a radial motion ofthe segments 4 of the nut 5 equivalent to about 0.3 mm. The completedisengagement between segments 4 and the screw 6 occurs with an about 1mm radial motion. With a rotational speed set at around 0.1 rpm, thepre-load is nullified after about 5 s and the complete disengagementafter about 7 s.

The use of piezoelectric micro-actuators, which—as mentioned—is notstrictly necessary, attains the further advantage of guaranteeing highblock torques in absence of power and generating a high repeatability ofthe imparted motion. Such repeatability characteristics enable themultiple use of devices according to the invention, that can be actuatedsimultaneously.

Lastly, the ability of the piezoelectric micro-actuators to develop hightorques at low rotational speed guarantees great precision andrepeatability, absence of clearances both when starting and invertingmotion, high block torque also obtainable in the absence of power, lowelectric energy consumption, small overall dimension, not particularlystrict construction tolerances, absence of lubrication, constructionsimplicity, high reliability and low costs.

Obviously, the construction details and the embodiments may widely varywith respect to what has been described and illustrated, withoutdeparting from the scope of protection of the invention as described inthe claims that follow. Thus, the block linear actuators could beremoved and the thrust linear actuators could also act from outside theelastic ring 16, instead of from inside, as exemplified in thepreviously mentioned document WO-2014/013442A1.

In the embodiment described up to now, and as illustrated in detail inFIGS. 15 and 16, the screw 6 directly cooperates with the segments 4 ofthe nut 5, which—as mentioned—are internally threaded like the screw 6.FIG. 15 shows the retaining configuration, in which the segments 4 aremutually radially approached engaging the thread of the screw 6, whileFIG. 16 represents the radially spaced apart configuration of thesegments 4 for the release of the screw 6.

FIGS. 17 and 18 show a variant, currently deemed the preferred one inthat advantageous for improving the mobility of the device when passingfrom the retaining configuration to the release configuration of thescrew 6. In such variant, there is provided an internally threadedbushing 30 permanently fastened on the screw 6 and formed with a radialengagement outer portion, for example constituted by an annular groove31. In this case, the blocking member 5 is formed by a triad of segments4 a provided with respective radial reliefs or teeth 4 b suitable toengage the radial engagement portion 31 of the bushing 30, in theretaining configuration of the device represented in FIGS. 17 and 19.FIGS. 18 and 20 show the release configuration in which the segments 4 aare radially spaced apart so as to disengage the bushing 30 and thusfree the screw 6.

This arrangement is advantageous in that the constraint threading of thescrew 6 is no longer interrupted and discontinuous. Furthermore, thebushing 30 may be formed with radial teeth or discontinuities 6 a which,being engaged with hindrance parts of the structure of the device, reactto the fastening torque applied by the segments 4 a thus hindering therotation of the screw 6. During the release step, the bushing 30 may beprecisely guided by the structure of the device, and—in addition—theaxial load applied in the bushing 30 by the screw 6 may be transferredto the segments 4 a even through an inclined plane 6 b, thus beingaxially and radially disassembled as schematised in FIG. 21. The axialposition of such inclined plane and the inclination thereof may beselected so as to minimise the radial loads acting on the articulationpins 9, 10 of the connecting rod and crank elements 7, 8, so as tofurther reduce the mobility required to pass from the retainingconfiguration to the release configuration. For example, it wasascertained that a 15° angle and a 0.13 coefficient of friction generatea substantially null radial load on the pins 9, 10.

The general configuration of the device, represented in FIGS. 19 and 20respectively in the retaining configuration and in the releaseconfiguration of the screw 6, does not differ from what has beendescribed above neither conceptually nor functionally.

As previously mentioned, the actuator for actuating the articulatedmechanism 3 formed by the connecting rods 7 and cranks 8 may beconstituted by a different motor suitable to generate the torquerequired for an angular travel of a few degrees, suitable to perform thecorresponding radial travel of the mobile segments 4 or 4 a of theblocking member 5. The two variants represented in FIGS. 22-25 and 26-29show two alternative examples in which the motor consists in elementsmade of shape memory material.

More in particular, the elements made of shape memory material consistin wires 32 for example made of NiTiNOL, each one of which may beextended, while in martensite phase, by a given percentage of theinitial length thereof, and installed in such condition. Thus, uponheating the wires 32 typically due to the Joule effect by flowing anelectrical current through them, the wires 32 will be subjected to atransformation in austenite phase during which they will recover theinitial length.

In the embodiment illustrated in FIGS. 22-23 and 24-25 the wires 32 aredistributed uniformly and obliquely connect the body of the device,constituted in this case by a radially outer stationary ring 34, with aradially inner rotatable ring 33 which simultaneously actuates theconnecting rod and crank units 7, 8, like in the previously describedembodiment. When the wires 32 are in martensite phase (FIGS. 22 and 24)the segments 4 or 4 a are closed and the screw 6 is held, while when thewires 32 are subjected to transformation in austenite phase (FIGS. 23and 25) the rotatable ring 33 is rotated relative to the stationary ring34 so as to simultaneously actuate the rod and crank units 7, 8 wherebythe segments 4 or 4 a are spaced apart thus freeing the screw 6.

In the case of FIGS. 26, 27 (martensite phase) and 28, 29 (austenitephase) the wires 32 made of shape memory material are instead arrangedin a discrete configuration, for example grouped in pairs so as toreduce the number of anchoring points on which the load that generatesthe rotation of the rotatable ring 33 with respect to the stationarybody 34 is exerted.

In any case, considering the same number of wire sections, torque andrequired travel, the number of shape memory wires 32, the length thereofand the distance thereof with respect to the centre of rotation of thedevice will preferably be equal in both configurations described above.In addition, in both configurations there will be provided for elasticmeans not illustrated, for example constituted by one or more helicalsprings, to guarantee the maintenance of the retaining configuration ofthe screw 6 in the steps prior to passing to the release configuration.

Actuation by means of shape memory wires is compatible with a high levelof repeated actuation capability which is necessary to carry out settingand calibration operations of the device.

1. A device for the hold-down and controlled release of space satellitesinstalled on launchers and loads installed on space satellites,comprising: a blocking member for the engagement of a screw, saidblocking member comprising segments radially displaceable from amutually approached retaining position to a mutually spaced-apartrelease position of the screw, a rotatable element coaxially rotatablewith respect to the blocking member, a plurality of connecting rods andcrank units operatively connecting said rotatable element with saidsegments of the blocking member, and an actuator for controlling arotation of said rotatable element and a simultaneous displacement ofthe segments of the blocking member between said retaining position andsaid release position via said plurality of connecting rods and saidcrank units.
 2. The device according to claim 1, wherein said blockingmember consists of a nut whose segments are suitable to directly engagesaid screw.
 3. The device according to claim 1, further comprising aninternally threaded bushing fastened on said screw and formed with anouter radial engagement portion, and wherein the segments of saidblocking member are suitable to engage said radial engagement portion ofsaid bushing.
 4. The device according to claim 1, wherein eachconnecting rod and crank unit defines a knee connected to said rotatableelement.
 5. The device according to claim 1, wherein said actuatorcomprises a first actuator of a plurality of actuators configured tocontrol the rotation of said rotatable element.
 6. The device accordingto claim 5 wherein said plurality of actuators comprises a plurality oflinear motors.
 7. The device according to claim 6 wherein said pluralityof linear motors comprises a plurality of piezoelectric micro-actuators.8. The device according to claim 5 wherein said plurality of actuatorscomprises a thrust actuator and a blocking actuator.
 9. The deviceaccording to claim 5, wherein said rotatable element is an elastic ringforcedly positioned between angularly spaced stationary retaining memberand said plurality of actuators, said plurality of actuators configuredto exert a radial thrust on said elastic ring between said stationaryretaining members to carry out sequential localised micro-deformationsthereon.
 10. The device according to claim 9, wherein said plurality ofactuators further comprise blocking linear motors, said blocking linearmotors configured to act on the elastic ring, radially arranged at saidstationary retaining means and synchronously actuated with said thrustlinear motors.
 11. The device according to claim 5, wherein saidplurality of actuators consist of elements made of shape memorymaterial.
 12. The device according to claim 11, wherein said rotatableelement is a ring and said elements made of shape memory materialconsist of wires which obliquely interconnect said ring with astationary body.
 13. The device according to claim 12, wherein saidwires are made of shape memory material are arranged according to adistributed configuration.
 14. The device according to claim 12, whereinsaid wires are made of shape memory material are arranged according to adiscrete configuration in groups.
 15. The device according to claim 1,further comprising means for detecting a pre-load of the screw.
 16. Thedevice according to claim 1, wherein said angularly spaced stationaryretaining members comprise locking pins.
 17. The device according toclaim 1, wherein said angularly spaced stationary retaining memberscomprise an annular element.
 18. The device according to claim 1 whereinsaid actuator is configured to exert a radial thrust on said rotatableelement to carry out a localised micro-deformation thereon.